The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to compositions and methods for treating subterranean formations, in particular, compositions and methods for cementing and completing wells which may be subjected to extreme dynamic stresses or into which aggressive fluids such as carbon dioxide or hydrogen sulfide are injected, stored or extracted.
During the construction of subterranean wells, it is common, during and after drilling, to place a tubular body in the wellbore. The tubular body may comprise drillpipe, casing, liner, coiled tubing or combinations thereof. The purpose of the tubular body is to act as a conduit through which desirable fluids from the well may travel and be collected. The tubular body is normally secured in the well by a cement sheath. The cement sheath provides mechanical support and hydraulic isolation between the zones or layers that the well penetrates. The latter function is important because it prevents hydraulic communication between zones that may result in contamination. For example, the cement sheath blocks fluids from oil or gas zones from entering the water table and polluting drinking water. In addition, to optimize a well's production efficiency, it may be desirable to isolate, for example, a gas-producing zone from an oil-producing zone.
The cement sheath achieves hydraulic isolation because of its low permeability. In addition, intimate bonding between the cement sheath and both the tubular body and borehole is necessary to prevent leaks. However, over time the cement sheath can deteriorate and become permeable. Alternatively, the bonding between the cement sheath and the tubular body or borehole may become compromised. The principal causes of deterioration and debonding include physical stresses associated with well operations, tectonic movements, temperature changes and chemical deterioration of the cement.
The pressure inside well casing may increase or decrease as the fluids change or as additional pressure is applied to the well, such as when drilling fluid is being replaced by a completion fluid or during a stimulation operation. The cement may also be subjected to stresses which are dynamic in nature either because they occur over a very short time period or because they are repetitive. For example, perforating does not just cause an over-pressure of a few hundred bars inside the well that dissipates in the form of a shock wave. In addition, perforating creates a shock when the projectile penetrates the cement and the shock subjects the zone surrounding the wellbore to large forces extending of a length of a few meters.
Casing expansion arising from large temperature increases experienced during thermal recovery processes such as cyclic steam stimulation (CSS) or steam assisted gravity drainage (SAGD) may impart large stresses to the cement sheath.
Another process that creates dynamic stresses in the cement is the cutting of cemented casing to create a sidetrack. Milling the steel over a depth of a few meters followed by drilling the sidetrack subjects the cement to shock and vibration that may cause irreversible damage.
Persons skilled in the art recognize that the risk of cement-sheath rupture depends on the thermoelastic properties of the casing, the cement, and the formation that surrounds the well. A detailed analysis of the mechanisms leading to rupture of the cement sheath has shown that the risk of rupture of a cement sheath following an increase in pressure and/or temperature in the well is directly linked to the tensile strength of the cement and is attenuated when the ratio of the tensile strength RT of the cement over its Young's modulus E is increased.
One aim of the present disclosure is to provide well cements that are reinforced with elastomers. The elastomer particles improve the flexibility of the set cement and the ability of the cement sheath to withstand physical stresses such as those described above.
Cements containing elastomers may also be applicable to wells into which carbon dioxide is injected (e.g. during Enhanced Oil Recovery technique), in which carbon dioxide is stored or from which carbon dioxide is recovered. In addition, there are some oil and gas wells whose reservoirs naturally contain carbon dioxide.
A relatively new category of wells involving carbon dioxide is associated with carbon-sequestration projects. Carbon sequestration is a geo-engineering technique for the long-term storage of carbon dioxide or other forms of carbon, for various purposes such as the mitigation of “global warming.” Carbon dioxide may be captured as a pure byproduct in processes related to petroleum refining or from the flue gases from power plants that employ fossil fuels. The gas is then usually injected into subsurface saline aquifers or depleted oil and gas reservoirs. One of the challenges is to trap the carbon dioxide and prevent leakage back to the surface; maintaining a competent and impermeable cement sheath is a critical requirement. Certain elastomers swell in the presence of carbon dioxide and, should the cement sheath suffer damage in the form of cracks, fissures or the like, the elastomer particles may swell and seal off the damage. Such cement systems are known in the art as “self-healing cements.” Similarly, there are elastomers known in the art that swell in the presence of hydrocarbons and hydrogen sulfide.
Elastomers known in the art for creating flexible or self-healing cements tend to have low densities; consequently, the slurry-density range of the cements may be limited to about 1920 kg/m3 (16.0 lbm/galUS). Weighting materials such as hematite, ilmenite, barite and manganese tetraoxide may be added to increase the slurry density. With elastomers heretofore used in the art, the maximum slurry density attainable for flexible or self-healing cements containing weighting materials may be about 2280 kg/m3 (19.0 lbm/galUS).